The Mesopotamians believed that the earth is the center of the universe. The motion of stars was explained by Greek thinkers Aristotle and Ptolemy as rotating crystal spheres. Although Greek astronomer Aristarchus of Samos maintained that the earth revolves around the sun, the concept of the earth as the center of the universe held until Polish astronomer Nicolaus Copernicus proposed that the planets revolve around the sun, which he believed to be the center of the universe. He explained the motion of the stars by the rotation of the earth. English mathematician and physicist Sir Isaac Newton stated general laws of motion and gravitation, which he said are valid throughout the universe.

Thousands of galaxies lie outside the Milky Way galaxy that contains the solar system. Each galaxy contains hundreds of billions of stars. Most galaxies are receding from the Milky Way at several hundred kilometers per second. American astronomer Edwin Hubble discovered that more remote galaxies have proportionally higher recession velocities. The universe is expanding, carrying galaxies away from each other in all directions.